This invention relates to an electrochromic display device which utilizes a combination of two types of electrochromic materials one of which takes on color in its electrochemically oxidized state while the other takes on color in its electrochemically reduced state.
There are several types of electrochromic (EC) display devices for displaying letters and/or figures. In EC display devices of a relatively simple construction, only the display electrode comprises a suitably patterned film of an EC material such as Prussian blue or tungsten trioxide, whereas the counter electrode on the opposite side is a simple and uniformly formed transparent conductive film on a glass substrate. However, this type of EC display devices does not possess sufficient endurance to repeated drive because the EC film is easily deteriorated mainly by the influence of decomposition reaction of water contained in the electrolyte liquid which intervenes between the display and counter electrodes.
In another type of EC display device as shown in JP-A No. 58-30731 for instance, one of the display electrode and the oppositely arranged counter electrode comprises a first type of EC film which takes on color in its electrochemically oxidized state and the other comprises a second type of EC film which takes on color in its reduced state. A typical example of the first type EC materials is Prussian blue which assumes blue color by oxidation, and a typical example of the second type EC materials is WO.sub.3 which assumes blue color by reduction. In this type of EC display device the EC films on the two electrodes are formed in the same display pattern. In operation, electrochemical oxidation of the display electrode is accompanied by electrochemical reduction of the opposite electrode, and vice versa. Accordingly simultaneous coloration and simultaneous bleaching of the two electrodes take place. However, conventional EC display devices of this type have disadvantages in several respects. First, not only the EC film on the display electrode but also the EC film on the counter electrode need to be formed by using a precise and troublesome patterning technique. Furthermore, in assembling the front and back substrates respectively provided with the display electrode and the counter electrode into a display device cell it is necessary to set the patterned EC film of the display electrode very accurately opposite to the identically patterned EC film of the counter electrode. These requirements offer difficulties in improving productivity and reducing production cost. Besides, in practical use of the EC display device the pattern of the letters or figures is seen as double image depending on the angle of sight since both of spaced two EC patterns exhibit color.